This invention relates to an ink-jet printing device, in which printing is carried out by inducing the selective emission of particles of an electrically conductive liquid ink through a nozzle from a container.
Various types of printers are known in which a selective ink-jet is produced by pressure pulses induced piezoelectrically, or by electric pulses inducing electrostatic ejection of droplets. These printers are generally very complicated and costly because of problems in the rapid drying of the droplets. It has therefore been sought to produce emission of the ink in other ways in order to ensure good penetration of the ink and quick drying.
In a known device (U.S. Pat. No. 2,143,376), the ink is contained in a conductive vessel of frusto-conical shape, of which the minor base, disposed upwards, is open for emission of the ink. The device comprises a point electrode which is disposed above the paper and is excited so as to cause the ink particles to be electrostatically attracted towards the electrode. This attraction is said to be favoured by a state of agitation of the surface of the liquid, and by its vaporisation caused by any electrical discharges created between the electrode and container. This method has various drawbacks, both because it is difficult to keep the ink at the level of the opening without marking the paper, and because of the difficulty producing consistent electrical discharges and the damage caused by them to the paper.
A printing device has also been proposed in which the ink is kept at a predetermined level in a tube having its opening facing upwards. Two electrodes are inserted into the tube so that they are disposed in the same horizontal plane, and remain immersed under a predetermined depth of ink. Ink emission is produced by instantaneous vaporisation of a portion of ink inside the nozzle at the level of the electrodes, so as to hurl the overlying layer of ink against the paper. In particular, in a modification of this device (U.S. Pat. No. 3,177,800) the ink is electrically nonconductive, and instantaneous vaporisation is produced by a breakdown in the dielectric properties of the ink, this inducing a spark between the electrodes.
In a further modification of the device comprising two electrodes immersed in the ink (U.S. Pat. No. 3,179,042), the ink is electrically conductive with a high electrical resistance, and is preheated to a temperature slightly lower than its boiling point. On exciting the two electrodes by means of a voltage pulse, current passes through the ink to produce instantaneous heat which vaporises a portion of ink, so expelling the overlying ink.
Both these modifications of the device with two immersed electrodes have the drawback of requiring a tube of considerable diameter to house the electrodes, so that it is not possible to obtain dots which are sufficiently small for a high definition printer.
A printing device has also been proposed (FR No. 2 092 577) in which the tube has a horizontal axis, and is connected to the bottom of a container of electrically non-conductive ink. Coaxially to the tube there is disposed an electrode in the form of a pointed needle, while into the free end of the tube there is inserted a metal sleeve which reduces the tube diameter and forms the second electrode. Because of its pressure, the ink normally fills the electrode, so that both the electrodes are immersed. On exciting the two electrodes, a spark is struck in the liquid between the two electrodes due to breakdown of the dielectric, and causes instantaneous vaporisation of parts of the ink between the electrodes, with the expulsion through the sleeve of the ink contained therein.
This device has the drawback that because of the ink pressure, particularly after relatively long intervals of inactivity, the ink tends to leak from the sleeve, whereas the spark passing through the ink causes undesirable physical-chemical transformations.
Finally, a printing device has been proposed (GB No. 2 007 162) in which in order to emit an ink droplet from a nozzle, an ink vapour bubble is created inside a nozzle by means of an electrothermal transducer disposed outside the nozzle. This device generates a large quantity of heat which has to be quickly eliminated in order to ensure repeatability of the phenomenon. In addition, only one droplet is generated each time, so that it has the same drawbacks as devices in which the ink jet is generated by piezoelectric or electrostatic means.
The object of the present invention is to provide a selective ink-jet printing device which prevents deterioration of the ink, and which ensures the printing of indelible marks which are immediately dried.
This problem is solved by the printing device according to the invention, which is characterised in that the container is insulating, the ink is kept under such a pressure such as to form a concave meniscus in the nozzle, and the printing of a dot is carried out by a voltage pulse between a first electrode which is in contact with the ink in the contaniner and a second electrode which is disposed outside the nozzle, so as to create excitation of the meniscus and an electric current in the ink in the nozzle which cause the expulsion of a spray constituted by a plurality of ink particles.